In recent years, the widespread use of portable devices, such as laptop personal computers and mobile phones, has led to the growing demand for batteries serving as power sources of the portable devices. There has been an increasing demand for, in particular, small and lightweight secondary batteries that have high energy density and can be repeatedly charged and discharged. There has recently been a growing new demand for such secondary batteries as power sources for driving, for example, electric tools, hybrid cars, and electric vehicles.
To address such a demand, nonaqueous electrolyte secondary batteries typified by lithium ion secondary batteries have been actively researched and developed. As performance is enhanced, and the output power of devices is improved, the energy of each of nonaqueous electrolyte secondary batteries has been increasing, and the amount of heat generated under abnormal conditions has increased.
Abnormal heat generation in a battery means, for example, heat generation caused by an internal short circuit in the battery or by overcharging. In particular, when a battery can and an electrode group are deformed with foreign matter, for example, by inserting a nail into a corresponding battery to create an internal short circuit between a positive electrode and a negative electrode in the electrode group, the amount of heat generated by a short circuit current passing through a positive electrode active material increases. Thus, the safety of such a battery is problematic.
To address such an internal short circuit, PATENT DOCUMENTS 1-3 each describe a nonaqueous electrolyte secondary battery including a battery can in which a wound electrode group is housed. In the nonaqueous electrolyte secondary battery, a current collector exposed portion of a positive electrode and a current collector exposed portion of a negative electrode facing the current collector exposed portion of the positive electrode each have a length of greater than or equal to one turn in a winding direction of the electrode group. In particular, in a situation where the current collector exposed portions are formed in an outermost portion of the electrode group, even when an internal short circuit is created, for example, by inserting a nail into the battery, the resistance of the short circuit path between the current collector exposed portions of the positive and negative electrodes formed in the outermost portion is less than that of the short circuit path between portions of active materials corresponding to an inner portion of the electrode group. Thus, short circuit current flows locally through a short circuit path between the current collector exposed portions. Consequently, the sharp increase in battery temperature can be reduced. Furthermore, if such a short circuit path between the current collector exposed portions is distributed in the electrode group, the sharp increase in battery temperature can be further reduced.